Barycenter kinematics in Local Group analogues

Abstract

We present a new way to identify systems similar to our Local Group (LG) of galaxies in cosmological simulations. Our method uses as a new constraint the speed and direction of the LG's center of mass, which we can measure accurately from cosmic microwave background data. When we apply these criteria to different cosmological simulations and compare the results with traditional selection methods, we find statistically significant differences. Our approach produces simulated galaxy pairs where the relative M31 velocity is less radial (2 percent to 7 percent difference over the mean) and more tangential (1 percent to 3 percent difference over the mean) than in cases that do not take into account the barycenter speed. The radial change pattern appears consistently across all cosmological models we test, showing that matching the observed barycenter velocity has a measurable effect when modeling and interpreting Local Group-like systems.

Figures (8)

• Figure 1: Diagram showing the alignment between key quantities used to select LG analogues. $\mathbf{v}_b$ represents the barycenter velocity of the LG and $\mathbf{r}$ shows the position of M31 relative to the MW.
• Figure 2: Left panel: cumulative distribution of $\mu$, the cosine of the angle between the barycenter velocity of the LG and the position vector of M31 relative to the MW. We also plot a uniform distribution from -1 to 1 for comparison Right panel: Cumulative distribution for the barycenter speed $v_b$ of the Local Group.
• Figure 3: Mean velocity components of M31 relative to the MW as a function of $v_b$ and $\mu$ in the base cosmology (c000) for the traditional sample. The top panel shows $v_r$ and the bottom panel shows $v_t$
• Figure 4: Mean values of M31 velocity distributions in all cosmologies (top panel shows $v_r$ and the bottom panel $v_t$). The main y-axis corresponds to the mean values of the velocities for traditional and realistic pairs. The secondary y-axis shows the percentage difference in mean values of realistic pair distributions compared to the mean of traditional pair distributions in each cosmology. Dashed lines and shaded regions represent the average and standard deviation of the mean values of traditional pair distributions for all cosmologies
• Figure 5: Statistical significance of the differences between realistic and traditional selection criteria for radial velocity $v_r$ (top panel) and tangential velocity $v_t$ (bottom panel). The $p$-values shown are from two-sample Kolmogorov-Smirnov tests across all cosmologies.